Downloading to a synced folder drastically slows the internet down

Windows 10 on both machines. 16GB ram, Synctrazor on both machines.

To be sure i was not going crazy, i tested this out before posting. If i download a file, such as a 1.5GB ISO image file, to my synced folder, my download speeds go to 18Kb/s and it says 17 hours remaining. I let the download run for a minute to see if it would speed up. It never did. As soon as i switch to a different folder on my laptop, such as the downloads folder, which is not synced, i get my normal 5MB/s speeds and 13 minutes of remaining time.

Even if my PC at home is downloading the information, it should not drop to extremely low speeds?

Edit: just tested again, and same issue. very slow… I am connected to a public EDUROAM wifi via the train i am on…


In this specific case, my guess would be that the train’s Wi-Fi connection is simply to weak for you to be able to both download the file and also upload the partially downloaded content to the other computer at the same time.

What are you downloading the file with though? If it’s a browser or similar, you may want to try adding its partial file extensions (e.g. *.crdownload in the case of Chromium) to your ignore patterns and then see whether there’s any difference.

The other culprit could be when using an HDD where the disk is too slow to read and write at decent speed simultaneously. However, seeing “16GB ram”, I’m assuming that you’re on an SSD here.


Adding to what @tomasz86 already mentioned…

Wi-Fi, at least not the current mainstream generations, isn’t full-duplex. Wired connections such as UTP Ethernet have pairs of transmit and receive lines that allow a network adapter to send and receive at the same time while a Wi-Fi transceiver alternates between sending and receiving.

It’s also likely that the train uses wireless repeaters to get the signal from car to car until it reaches a router connected to a cell modem. The repeaters reduce bandwidth by ~50%, have the same limitations sending and receiving, plus cellular/satellite networks prioritize download over upload speeds so any uploads impact downloads more than a full-duplex network.


i am using Edge browser and the default download manager they use.

After i wrote the post, i thought of the trains wi-fi being an issue. Ill have to test it at another location and see, but i ride the train like 6 hours a day haha.

Yes, i am on an SSD drive for sure. used to be mechanical until windows 10 killed the drive. When you would go through the actual windows menu, hit power, then sleep, then close the lid because you think its asleep, then windows decide to wakeup with the lid closed and the drive is spinning while im walking with it in my backpack… good way to kill a mechanical drive haha

I also commute by trains quite a bit myself, but from my personal experience, the only reliable way to use the Internet while on board is to use my own mobile data plan :confused:. The provided Wi-Fi has always been either too slow or unstable (or simply not working at all despite being “connected”).

Yeah, Windows 8(.1) was the last version that worked decently on mechanical drives :slightly_frowning_face:. Windows 10 and later has too many I/O requests in comparison to the previous versions which leads to constant 100% disk usage and thus extreme slowness… unless we’re talking about 10 or 15,000 rpm drives, of course, as those would still deliver decent performance.

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Syncthing by default runs with some sort of low I/O priority. I’m not sure how that works in Windows, but I wouldn’t be surprised if it meant that if someone else is writing to the same disk/folder/whatever it causes Syncthing to back off for the duration.

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Are you sure about this? From my testing, the CPU priority is indeed set to below normal (when using, but the I/O priority is at its default normal, i.e. not affected by the configuration setting.

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I thought setting the priority class affected I/O as well, but it looks like it doesn’t so no, I’m not sure. :smiley:

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Yeah, I think Windows requires setting the three priorities separately — CPU, I/O, and RAM.

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